### OAuth2 purpose

A way for the `user` to tell `google` to give an access token to `xorismesiti.gr` app

<br><br><br>

### OAuth2 Standar Flow:

1. **User** clicks button "Login with Google" on your platform `xorismesiti.gr`
2. **Authorization Request**: Button redirects user to Google's authorization endpoint `accounts.google.com/o/oauth2`
3. **User Login and Consent**: User login to Google and grants permissions.
4. **Authorization Code Response**: Google redirects user back to your app `xorismesiti.gr/callback` with an authorization `code`.
5. **Access Token Request**: App exchanges the authorization `code` for an `access_token`.
6. **Access Protected Resources**: App uses the `access_token` to fetch the user's Google profile and email from `googleapis.com/oauth2`
7. **Token Refresh** (Optional): If the `access_token` expires, app uses the `refresh token` to get a new `access_token`.

<br><br><br>

### OAuth2 Frontend/Backend Flow:

**Frontend**
1. **Redirect** the user to Google's OAuth authorization endpoint `accounts.google.com/o/oauth2`
2. **Get** the authorization `code` after Google redirects back to the frontend `xorismesiti.gr/callback`
3. **Send** the authorization `code` to the backend for `token` exchange.

**Backend**
1. **exchange** the authorization `code` for an `access_token` and `refresh token`
2. **fetch** user profile data from from `googleapis.com/oauth2` using the `access_token` 
3. **Store** the `tokens` securely in session (front) or a database (back)
4. **Refresh** the `access_token` if it expires

<br><br><br>





# 1. [Frontend] Request Authorization code

1. A button "Login with Google" redirects the user to the Google's authorization endpoint `accounts.google.com/o/oauth2/v2/auth`
2. After the redirection, the user will log in to Google and grant permissions (if they haven’t already).
3. Google will redirect the user back to your redirect_uri `https://xorismesiti.gr/callback` with an authorization code `?code=`

*Security: the state string should be validated upon receiving the response from Google, as it ensures that the response corresponds to the request.*

<details>
<summary><h3>Frontend HTTP GET Request to Google</h3></summary>

```sh
GET https://accounts.google.com/o/oauth2/v2/auth?
    response_type=code&
    client_id=ABC34JHS9D&
    redirect_uri=https://xorismesiti.gr/callback&
    scope=email%20profile&
    state=xyz123
```

- `response_type=code`: This indicates you're using the "authorization code" flow.
- `client_id`: Your Google API client ID. Created by the developers of the `xorismesiti.gr` on the google API console.
- `redirect_uri`: The URI Google will redirect to after the user consents.
- `scope`: The permissions you're requesting (e.g., email, profile).
- `state`: A random string to protect against CSRF attacks.

</details>

<details>
<summary><h3>Google HTTP Response to Frontend </h3></summary>

```bash
HTTP/1.1 302 Found
Location: https://xorismesiti.gr/api/auth/callback?code=4/0AX4XfWgyVyz-uT8k7WiyLg0Q&state=xyz123
Content-Type: text/html; charset=UTF-8
Content-Length: 0
```

</details>

<details>
<summary><h3>Frontent Code</h3></summary>

```js
// Redirect to Google's OAuth 2.0 endpoint when user clicks login
const loginWithGoogle = () => {
  const clientId = 'ABC34JHS9D';  // Replace with your actual Google Client ID
  const redirectUri = 'https://xorismesiti.gr/api/auth/callback'; // Backend URL where Google will send the user after login
  const scope = 'email profile';  // Scopes you're requesting (email, profile, etc.)
  const state = 'random-state-value';  // For CSRF protection
  
  const googleAuthUrl = `https://accounts.google.com/o/oauth2/v2/auth?response_type=code&client_id=${clientId}&redirect_uri=${redirectUri}&scope=${scope}&state=${state}`;
  
  window.location.href = googleAuthUrl; // Redirect user to Google
};
```

</details>

<br><br><br>





# 2. [Frontend] Receive Authorization Code

Now that the frontend has the Authorization `code` on th callback url https://xorismesiti.gr/api/auth/callback`?code=AAAABCX4XfWgyVyziyLg0QHHHHH` it can send it to the backend with POST to `xorismesiti.gr/api/auth/exchange-token`, in order the backend to exchange the `code` for an `access_token` and optionally an `refresh_token`

<details>
<summary><h3>Frontend HTTP POST Request to Backend</h3></summary>

```bash
POST https://xorismesiti.gr/api/auth/exchange-token
Content-Type: application/json

{
  "code": "AAAABCX4XfWgyVyziyLg0QHHHHH"
}
```

</details>

<details>
<summary><h3>Backend HTTP Response to Frontend</h3></summary>

```json
{
  "access_token": "ya29.a0AfH6SMC8Op6zXZkHi2XITkDoOVzYXt3hTY6sny54UlWlxrnKlX5Xv78is7BEHekVX-VoA",
  "token_type": "Bearer",
  "expires_in": 3600,
  "refresh_token": "1//04d5XHqmn6Hdy3wTf5OYDP1SyBa74zEFURjddQ2A1cFw78PY13pQyWhlD2A6XhDQtKlrjAqU4kS3vGdMvckw",
  "scope": "email profile"
}
```

</details>

<details>
<summary><h3>Frontend Code</h3></summary>

```js
// On the backend callback URL, the frontend receives the authorization code
import { useEffect } from 'react';
import { useRouter } from 'next/router';

const Callback = () => {
  const router = useRouter();
  
  useEffect(() => {
    const { code, state } = router.query;
    
    // Send the authorization code to the backend for token exchange
    fetch('/api/auth/exchange-token', {
      method: 'POST',
      body: JSON.stringify({ code }),
      headers: {
        'Content-Type': 'application/json',
      },
    })
    .then(response => response.json())
    .then(data => {
      // Handle success (store token, update UI, etc.)
      console.log(data);  // Typically, you'll store the access token here or manage the user session.
      router.push('/dashboard');  // Redirect the user to their dashboard or home page.
    })
    .catch(error => {
      console.error('Error exchanging token:', error);
    });
  }, []);
  
  return <div>Loading...</div>;
};

export default Callback;
```

</details>

<br><br><br>





# 3. [Backend] Exchange Code with Token 

1. The backend **receives** the authorization `code` form frontend (Frontend POST at `xorismesiti.gr/api/auth/exchange-token`)
2. The backend **POST** Authorization `code` to Google API
3. The Google API respond to backend POST with the tokens `access_token` and `refresh_token`
4. The backend **respond** to frontend with the tokens (respond to frontend POST at `xorismesiti.gr/api/auth/exchange-token`)

*Security: The backend never expose the client_secret to the frontend. This step should always be handled on the backend.*

<details>
<summary><h3>Backend HTTP POST Request to Google</h3></summary>

```sh
POST https://oauth2.googleapis.com/token
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&
code=AAAABCX4XfWgyVyziyLg0QHHHHH&
redirect_uri=https://xorismesiti.gr/callback&
client_id=ABC34JHS9D&
client_secret=PASS1234
```

- `HTTP Method`: POST
- `URL`: https://oauth2.googleapis.com/token
- Headers
    - `Content-Type`: application/x-www-form-urlencoded
- Body
    - `grant_type`=authorization_code: This specifies the grant type.
    - `code`: The authorization code you received in the previous step.
    - `redirect_uri`: The same redirect URI used in the authorization request.
    - `client_id`: Your Google API client ID.
    - `client_secret`: Your Google API client secret (which should be kept secure).

</details>

<details>
<summary><h3>Google HTTP Response Backend</h3></summary>

```json
{
    "access_token": "ya29.a0AfH6SMC8Op6zXZkHi2XITkDoOVzYXt3hTY6sny54UlWlxrnKlX5Xv78is7BEHekVX-VoA",
    "token_type": "Bearer",
    "expires_in": 3600,
    "refresh_token": "1//04d5XHqmn6Hdy3wTf5OYDP1SyBa74zEFURjddQ2A1cFw78PY13pQyWhlD2A6XhDQtKlrjAqU4kS3vGdMvckw",
    "scope": "email profile"
}
```

</details>

<details>
<summary><h3>Backend Code:</h3></summary>

```js
const express = require('express');
const axios = require('axios');

const app = express();
app.use(express.json()); // To parse JSON bodies

const clientId = 'YOUR_GOOGLE_CLIENT_ID'; // Google Client ID
const clientSecret = 'YOUR_GOOGLE_CLIENT_SECRET'; // Google Client Secret
const redirectUri = 'https://xorismesiti.gr/api/auth/callback'; // Must match the one used in frontend

// Handle token exchange
app.post('/api/auth/exchange-token', async (req, res) => {
  const { code } = req.body;

  try {
    const response = await axios.post('https://oauth2.googleapis.com/token', null, {
      params: {
        code,
        client_id: clientId,
        client_secret: clientSecret,
        redirect_uri: redirectUri,
        grant_type: 'authorization_code',
      },
    });
    
    const { access_token, refresh_token, expires_in } = response.data;

    // Optionally, store the tokens in a secure location (e.g., session, database)
    // For now, send them back to the frontend (not recommended for production)
    res.json({ access_token, refresh_token, expires_in });
  } catch (error) {
    console.error('Error exchanging authorization code for token:', error);
    res.status(500).json({ error: 'Failed to exchange authorization code for access token' });
  }
});

// Start the server
app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});
```

</details>

<br><br><br>





# 4. [Frontend] Use the Token

Once the backend exchanges the `code` for the `access_token`,

incude these tokens to POST reponse recieved from frontend,

so that the frontend can use it to make authenticated requests to the backend or Google APIs

<details>
<summary><h3>Frontend HTTP GET Request to Backend</h3></summary>

```bash
GET https://xorismesiti.gr/api/user-profile
Authorization: Bearer access-token-from-backend
```

</details>

<details>
<summary><h3>Frontend HTTP Response from Backend</h3></summary>

```json
{
    "sub": "1234567890",
    "name": "John Doe",
    "given_name": "John",
    "family_name": "Doe",
    "profile": "https://plus.google.com/1234567890",
    "picture": "https://lh3.googleusercontent.com/a-/AOh14GgIXXl5JXzW0c1Szbl-e1Jch1vhl5rHhH65vlK6J5g5PqkGjj1O0p3t8bgVEOykQ6ykFSQ=s96",
    "email": "john.doe@example.com",
    "email_verified": true,
    "locale": "en"
}
```

</details>

<details>
<summary><h3>Frontend Code</h3></summary>

```js
// Store tokens and expiration time after receiving them from the backend
const storeTokens = (access_token, refresh_token, expires_in) => {
  const expirationTime = Date.now() + expires_in * 1000; // expires_in is in seconds, so multiply by 1000 to get ms
  localStorage.setItem('access_token', access_token);
  localStorage.setItem('refresh_token', refresh_token);
  localStorage.setItem('token_expiration', expirationTime);
};

// Function to check if the access token has expired
const isTokenExpired = () => {
  const expirationTime = localStorage.getItem('token_expiration');
  return Date.now() > expirationTime;
};

// Function to refresh the access token using the refresh token
const refreshAccessToken = async () => {
  const refresh_token = localStorage.getItem('refresh_token');
  try {
    const response = await fetch('/api/auth/refresh-token', {
      method: 'POST',
      body: JSON.stringify({ refresh_token }),
      headers: {
        'Content-Type': 'application/json',
      },
    });

    const data = await response.json();
    if (data.access_token) {
      // Store the new access token and expiration time
      storeTokens(data.access_token, data.refresh_token, data.expires_in);
      return data.access_token;
    }
  } catch (error) {
    console.error('Error refreshing token:', error);
    return null; // Handle error appropriately (e.g., logout user or prompt to log in again)
  }
};

// Function to get the access token, either from localStorage or by refreshing it if expired
const getAccessToken = async () => {
  if (isTokenExpired()) {
    console.log('Access token expired. Refreshing...');
    const newAccessToken = await refreshAccessToken();
    return newAccessToken;
  } else {
    return localStorage.getItem('access_token');
  }
};

// Example usage: Get the access token and use it for an authenticated API request
const fetchUserProfile = async () => {
  const access_token = await getAccessToken();
  
  if (!access_token) {
    console.error('No valid access token found. User might need to log in.');
    return;
  }

  // Now you can use the access token to make an authenticated API request
  try {
    const response = await fetch('/api/user-profile', {
      headers: {
        'Authorization': `Bearer ${access_token}`,
      },
    });
    
    const userData = await response.json();
    console.log(userData);
  } catch (error) {
    console.error('Error fetching user profile:', error);
  }
};

// Example call to fetch user profile
fetchUserProfile();
```

</details>

<br><br><br>









# 5. [Backend] Fetch User Data

With the access token obtained in the previous step, 

your platform can now use it to fetch the user's Google profile and email information. 

The token is included in the Authorization header of the request.

<details>
<summary><h3>HTTP GET Request</h3></summary>

```sh
GET https://www.googleapis.com/oauth2/v3/userinfo
Authorization: Bearer ya29.a0AfH6SMC8Op6zXZkHi2XITkDoOVzYXt3hTY6sny54UlWlxrnKlX5Xv78is7BEHekVX-VoA
```

- URL: https://www.googleapis.com/oauth2/v3/userinfo
- HTTP Method: GET
- Headers:
- Authorization: Bearer {access_token}: The access token obtained in step 5.

</details>

<details>
<summary><h3>HTTP Response</h3></summary>

```json
{
    "sub": "1234567890",
    "name": "John Doe",
    "given_name": "John",
    "family_name": "Doe",
    "profile": "https://plus.google.com/1234567890",
    "picture": "https://lh3.googleusercontent.com/a-/AOh14GgIXXl5JXzW0c1Szbl-e1Jch1vhl5rHhH65vlK6J5g5PqkGjj1O0p3t8bgVEOykQ6ykFSQ=s96",
    "email": "john.doe@example.com",
    "email_verified": true,
    "locale": "en"
}
```



</details>

<details>
<summary><h3>Backend GET Code</h3></summary>

```js
app.get('/api/user-profile', async (req, res) => {
  const accessToken = req.headers['authorization'].split(' ')[1]; // Extract token from Authorization header
  
  try {
    const response = await axios.get('https://www.googleapis.com/oauth2/v3/userinfo', {
      headers: {
        Authorization: `Bearer ${accessToken}`,
      },
    });
    
    // Send the user profile data to the frontend
    res.json(response.data);
  } catch (error) {
    console.error('Error fetching user profile:', error);
    res.status(500).json({ error: 'Failed to fetch user profile' });
  }
});
```

</details>

<br><br><br>







# 6. [Backend] Token Expiry and Refresh (Optional)

If the access token expires, 

your platform can use the refresh token (if provided) to obtain a new access token without requiring the user to log in again.


<details>
<summary><h3>HTTP POST Request</h3></summary>

```bash
POST https://oauth2.googleapis.com/token
Content-Type: application/x-www-form-urlencoded

grant_type=refresh_token&
refresh_token=refresh-token-from-backend&
client_id=YOUR_GOOGLE_CLIENT_ID&
client_secret=YOUR_GOOGLE_CLIENT_SECRET
```

- `URL`: https://oauth2.googleapis.com/token
- `HTTP` Method: POST
- `Headers`:
- `Content`-Type: application/x-www-form-urlencoded
- `Body` Parameters:
- `grant_type`=refresh_token: This indicates the refresh token flow.
- `refresh_token`: The refresh token obtained in step 5.
- `client_id`: Your Google API client ID.
- `client_secret`: Your Google API client secret.

</details>

<details>
<summary><h3>HTTP Response</h3></summary>

```json
{
    "access_token": "ya29.a0AfH6SMC8Op6zXZkHi2XITkDoOVzYXt3hTY6sny54UlWlxrnKlX5Xv78is7BEHekVX-VoA",
    "token_type": "Bearer",
    "expires_in": 3600
}
```

</details>

<details>
<summary><h3>Example Backend Code:</h3></summary>

```js
app.post('/api/auth/refresh-token', async (req, res) => {
  const { refresh_token } = req.body;

  try {
    const response = await axios.post('https://oauth2.googleapis.com/token', null, {
      params: {
        refresh_token,
        client_id: clientId,
        client_secret: clientSecret,
        grant_type: 'refresh_token',
      },
    });
    
    res.json(response.data); // Return new access token and refresh token
  } catch (error) {
    console.error('Error refreshing access token:', error);
    res.status(500).json({ error: 'Failed to refresh access token' });
  }
});
```

</details>